Line data    Source code

       1             : // Copyright 2016 The Cockroach Authors.
       2             : //
       3             : // Licensed under the Apache License, Version 2.0 (the "License");
       4             : // you may not use this file except in compliance with the License.
       5             : // You may obtain a copy of the License at
       6             : //
       7             : //     http://www.apache.org/licenses/LICENSE-2.0
       8             : //
       9             : // Unless required by applicable law or agreed to in writing, software
      10             : // distributed under the License is distributed on an "AS IS" BASIS,
      11             : // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
      12             : // implied. See the License for the specific language governing
      13             : // permissions and limitations under the License.
      14             : 
      15             : package bytealloc
      16             : 
      17             : import "github.com/cockroachdb/pebble/internal/rawalloc"
      18             : 
      19             : // An A provides chunk allocation of []byte, amortizing the overhead of each
      20             : // allocation. Because the underlying storage for the slices is shared, they
      21             : // should share a similar lifetime in order to avoid pinning large amounts of
      22             : // memory unnecessarily. The allocator itself is a []byte where cap() indicates
      23             : // the total amount of memory and len() is the amount already allocated. The
      24             : // size of the buffer to allocate from is grown exponentially when it runs out
      25             : // of room up to a maximum size (chunkAllocMaxSize).
      26             : type A []byte
      27             : 
      28             : const chunkAllocMinSize = 512
      29             : const chunkAllocMaxSize = 512 << 10 // 512 KB
      30             : 
      31           2 : func (a A) reserve(n int) A {
      32           2 :         allocSize := cap(a) * 2
      33           2 :         if allocSize < chunkAllocMinSize {
      34           2 :                 allocSize = chunkAllocMinSize
      35           2 :         } else if allocSize > chunkAllocMaxSize {
      36           1 :                 allocSize = chunkAllocMaxSize
      37           1 :         }
      38           2 :         if allocSize < n {
      39           1 :                 allocSize = n
      40           1 :         }
      41           2 :         return rawalloc.New(0, allocSize)
      42             : }
      43             : 
      44             : // Alloc allocates a new chunk of memory with the specified length.
      45           2 : func (a A) Alloc(n int) (A, []byte) {
      46           2 :         if cap(a)-len(a) < n {
      47           2 :                 a = a.reserve(n)
      48           2 :         }
      49           2 :         p := len(a)
      50           2 :         r := a[p : p+n : p+n]
      51           2 :         a = a[:p+n]
      52           2 :         return a, r
      53             : }
      54             : 
      55             : // Copy allocates a new chunk of memory, initializing it from src.
      56           2 : func (a A) Copy(src []byte) (A, []byte) {
      57           2 :         var alloc []byte
      58           2 :         a, alloc = a.Alloc(len(src))
      59           2 :         copy(alloc, src)
      60           2 :         return a, alloc
      61           2 : }
      62             : 
      63             : // Reset returns the current chunk, resetting allocated memory back to none.
      64             : // Future allocations will use memory previously allocated by previous calls to
      65             : // Alloc or Copy, so the caller must know know that none of the previously
      66             : // allocated byte slices are still in use.
      67           2 : func (a A) Reset() A {
      68           2 :         return a[:0]
      69           2 : }